Internal combustion engines fluidly couple to exhaust aftertreatment systems that purify exhaust gases generated as byproducts of combustion. Exhaust aftertreatment systems may include oxidation catalysts, reduction catalysts, selective catalytic reduction catalysts and particulate filters. Byproducts of combustion may include unburned hydrocarbons, carbon monoxide, nitrides of oxide, which may be referred to as NOx molecules, and particulate matter. Operation may be monitored by one or more sensing devices that are disposed in the exhaust gas feedstream, including, e.g., a NOx sensor. Operation may also be determined employing simulation models that dynamically execute during operation.
Selective catalytic reduction catalysts (SCRs) may employ reductants for reducing NOx molecules to elemental nitrogen. One known reductant is urea, which may be transformed into ammonia (NH3) in an exhaust system. The reductant may be injected into the exhaust gas feedstream upstream of one or multiple selective catalytic reduction catalysts, and may be stored on a surface or otherwise captured for use in reducing NOx molecules to elemental nitrogen and water.
Signal output from a downstream NOx sensor may exhibit a cross-sensitivity between NOx molecules and NH3 molecules when disposed to monitor an exhaust gas feedstream downstream of an SCR. As such, known reductant injection control systems operating in an open-loop control scheme to control reductant injection may be operating in an underdosing or an overdosing condition, depending upon magnitude of NH3 exiting the SCR. An open-loop reductant control system may result in reduced performance of an SCR that is sensitive to hardware variation and may result in a false-positive output from a diagnostic monitoring routine for an SCR under certain operating conditions.